The present invention relates to a process for treating metal oxide powders or granules in order to make them more suitable for compacting and fritting.
More specifically, it relates to the treatment of metal oxides, which can have several valency states and can give rise to the formation of hydrated oxides.
In particular, such oxides can be constituted by uranium dioxide, or plutonium dioxide, used in the pure state or in mixtures, or mixed with thorium dioxide, i.e. oxides usable for the production of nuclear fuel pellets, which may also contain temporary neutron absorbents such as Gd.sub.2 O.sub.3, Eu.sub.2 O.sub.3, etc.
One of the standard processes for producing nuclear fuel pellets from powders of these oxides, consists of bringing the powder into pellet form by cold compression and then subjecting the thus obtained crude pellets to natural fritting at an appropriate temperature. Following fritting, the fritted pellets firstly undergo a grinding treatment, so that they have the requisite geometrical characteristics and then inspection operations, in order to check whether or not they are in accordance with specifications.
The performance of these different stages involves numerous more or less automated manipulations of the pellets in both the crude and fritted states, which causes problems due to the brittleness of the pellets in both these states. Thus, the pellets are sensitive to impacts occurring during transfers and manipulations and more particularly therefore on their sharp edges when in the crude state. These impacts lead to defects, or cracking of splintering of the material and these prejudicial effects are not cured by the fritting treatment. For example, in certain cases, approximately 15% of the fritted pellets have 1 to 4 mm.sup.2 surface cracks or chips, particularly on their edges.
Thus, during the final inspection, certain of these pellets are considered to be unacceptable and are discarded. They can be transformed into powder, which can then be recycled after an appropriate treatment. However, this recycling involves additional operations and leads to a slight reduction in the quality of the pellets obtained from the recycled powder.
It would therefore be advantageous to improve the mechanical behaviour of the crude pellets and in particular their stability and impact resistance, because they have a considerable influence on the quality of the finished product and on production efficiency. Thus, the surface faults mainly caused during the production of the crude pellet, can be aggravated by manipulations and subsequent operations, such as the grinding of the fritted pellets.
In order to improve the mechanical behaviour of the crude pellets, a small proportion of organic binder, e.g. polyvinyl alcohol is added to the oxide powder, which makes it possible to increase the tensile strength of the crude pellets from 0.3 to approximately 0.6 MNm.sup.-2 and to bring about a marked reduction in the number of pellets having surface cracks and chips, so that the damaged pellets only represent 8% of the total number produced. However, the need to use a binder leads to certain disadvantages because it is necessary to add the binder in aqueous solution and then remove it during prefritting, which leaves behind solid carbonaceous residues, which can be prejudicial to the satisfactory fritting of the pellets.
Consideration has also been given to subjecting the initial oxide powder to a treatment, which consists of cold compressing the powder in order to obtain tablets, then crushing the tablets to obtain granules and adding to the latter approximately 0.3% of organic binder, before cold compressing them to obtain better quality crude pellets.
However, this process also suffers from the disadvantage of requiring on the one hand, the addition of the organic binder which must then be removed during fritting and, on the other hand, the performance of supplementary compression and crushing stages.